Evidence for important role of peroxynitrite, a product of superoxide reaction with nitric oxide, in diabetic complications is emerging. Using new pharmacological agents i.e., peroxynitrite decomposition catalysts and protein nitration inhibitor, and iNOS-knockout mice, we obtained findings implicating peroxynitrite injury in toto and protein nitration in early experimental peripheral diabetic neuropathy (PDN). We found that 1) nitro- tyrosine (NT), a footprint of peroxynitrite injury, accumulates in peripheral nerve, spinal cord, and DRG neurons of streptozotocin (STZ)-diabetic and ob/ob mice; 2) STZ-diabetic iNOS-deficient mice do not develop nerve conduction deficits and have less severe sensory neuropathy compared with diabetic wild-type mice; and 3) peroxynitrite decomposition catalysts and, to a lesser extent, a protein nitration inhibitor, corrected nerve conduction deficits and sensory neuropathy in mice with Type 1 and Type 2 diabetes. We also found that Type 2 diabetic patients accumulate different amounts of nitrated proteins in peripheral blood monocytes, that monocyte NT concentration is ~ 75% greater in Type 2 diabetic subjects compared with non-diabetic group, and that skin production of NO, a precursor of peroxynitrite, is reduced by pioglitazone treatment. Others showed that increased plasma NT content correlated with endothelial dysfunction and redistribution of sudomotor responses, an early sign of sympathetic nerve dysfunction, in diabetic patients. The overall objective of this proposal is to dissect the roles of peroxynitrite and protein nitration in functional and morphological changes of advanced experimental PDN, and to determine if NT levels in serum, peripheral blood monocytes, and skin can be used as biomarkers of the presence, severity, and progression of PDN in human subjects with diabetes. The specific aims are: 1) evaluate if peroxynitrite decomposition catalyst and protein nitration inhibitor reverse functional, behavioral, and morphological changes of advanced PDN in STZ- diabetic and Akita mice; 2) assess the effect of blood glucose control on accumulation and disappearance of NT in tissue-sites of PDN, skin, and circulation; and 3) determine potential values of serum, peripheral blood monocyte, and skin NT levels as biomarkers of the presence, severity, and progression of PDN in human subjects with diabetes mellitus. The findings will advance our understanding of the pathogenesis of PDN, and may provide rationale for development of new therapeutics. They may also lead to identification of a new biomarker(s) of PDN with diagnostic and prognostic value.